Painting can always be messy, and spray painting can especially be troublesome with its overspray that seems to get everywhere. Finished houses have floors, carpets, walls, furniture, and such that need protection from paint spray if it's to be done inside. One way to protect them is to bag or cover the collateral objects, the other way is to remove or enclose the paint spraying in a booth.
Aside from the problems of overspray, painting big items like doors is not so easy. They are large, awkward, and sometimes very heavy. All sides and edges need to be painted, and that leaves nowhere to grip or support the door until it's dry.
What is needed is a way to paint items on-site and easily, to control any overspray, and to be able to pick up a wet item and to put it somewhere else to dry.